Multiple-luminescent layer improved lumen maintenance combination

ABSTRACT

A luminescent discharge lamp combination in which a plurality of distinct layers of different luminescent materials is provided. A layer of very high lumen maintenance material is disposed to be first impinged by the exciting ultraviolet radiations. This high maintenance layer is uniformly coated onto a layer of predetermined luminescent material which has a less favorable maintenance characteristic. The high maintenance layer serves to screen out the more damaging shorter wavelength radiation from the proper maintenance material.

United States Patent MULTIPLE-LUMINESCENT LAYER IMPROVED LUMEN MAINTENANCE COMBINATION 6 Claims, 1 Drawing Fig.

Fluorescent Lamps and Lighting," by Elenbaas et a1, chapter 11, section 2.3, pages 18- 23 and chapter 111, sections 3.9- 3.12, pages 58- 64, 1962, copy of book in A.U. 252.

Primary ExaminerRoy Lake Assistant Examiner-Palmer C. Demeo Atto r neys-A. T. Stratton, W. D. Palmer and Walter Sutcliff [52] US. Cl....; 313/109,

- 313/ 184 o [51] lnt.Cl H01j6l/44 ABSTRACT; A luminescent discharge |amp combination in [50] Field of Search 313/109, which a plurality of distinct layers of different luminescent 184 materials is provided. A layer of very high lumen maintenance material is disposed to be first impinged by the exciting ul- [56] References cued traviolet radiations. This high maintenance layer is uniformly UNITED STATES PATENTS coated onto a layer of predetermined luminescent material 2,103,085 12/1937 McKeag et a] 313/109 which has a less favorable maintenance characteristic. The 2,135,732 11/1938 Randall et a1... 313/109 high maintenance layer serves to screen out the more damag- 2,424,454 7/1947 Gordon 3 1 3/ 109 ing shorter wavelength radiation from the proper maintenance 2,452,518 10/1948 Burns 310 9 X 7 material.

PATENTED M1831 l5?! 3.602.757

WITNESSESI INVENTOR 9 Anselm Wochtel 2 av ATTORNEY MULTIPLE-LUMINESCENT LAYER IMPROVED LUMEN MAINTENANCE COMBINATION BACKGROUND OF THE INVENTION The fact that all luminescent materials suffer a loss of efficiency of emitted light over an operating lifetime is well known. For various materials this maintenance characteristic varies considerably. It is, of course, a necessity for a commercial lamp that the maintenance characteristic meet some minimum standard no matter how good the initial output may be.

It is generally accepted that the most important factor in phosphor maintenance in a mercury discharge combination is the amount of high energy, short wavelength radiations are impinging upon the phosphor layer. The phosphor can also degenerate from direct bombardment of mercury ions.

It is, of course, the aim in the design of a conventional fluorescent lamp to provide a well balanced spectral energy distribution of generally white light. That this can be achieved in the most efficient mode by providing a blend of phosphors having selected spectral energy distributions is taught in copending application Ser. No. 742,291 filed July 3, 1968, owned by the assignee of the present application.

It is an object of this invention to provide a luminescent combination having a high efficiency for generating light and a superior lumen maintenance. It is also preferable that the light output is generally a white light with a high color rendering capacity.

SUMMARY OF THE INVENTION It has been discovered that by providing a discharge lamp combination with plural coatings of distinct, finely divided, predetermined luminescent materials, with a first layer disposed on a substrate and a second layer coated on the second layer, and wherein the second layer of material has a substantially better lumen maintenance, the overall maintenance of the combination can be very much improved. The luminescent combination comprises a source of ultraviolet radiation, which radiation impinges -upon the plural luminescent coatings on a light transmissive substrate. The high maintenance layer upon which the ultraviolet radiations directly impinge screen a portion of the high energy, short wavelength radiations from the one or more layers over which the high maintenance layer is disposed.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a view partly in section of a fluorescent lamp embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the sole FIGURE, a standard low-pressure mercury vapor discharge device is shown which serves as a substrate for the luminescent layers or coatings l4, 16, which are disposed on the interior surface of the envelope 12. The envelope 12 is typically formed of soda-lime glass. The conductive leaddns 18 are hermetically sealed through the envelope and ter minate internally in the electrodes across which the discharge is maintained. The lamp typically includes a discharge sustaining fill of mercury and an insert starting gas as is well known in the art.

The first luminescent layer 14 serves as the first layer on the envelope 12. This first layer 14, is by way of example a blend of the compositions Sr .,(P0 Cl: Eu which is taught by copending application Ser. No. 768,455 filed Oct. 17, 1968, owned by the present assignee, and the luminescent composition comprising the fired reaction product of 1.7 moles Zn0, 0.3 mole MgF 0.6 mole 6e0 0.4 mole Si0,, and 0.03 mole MnC0 described in copending application Ser. No. 834,971, filed concurrently herewith and owned by the present assignee. The phosphor preferably is stabilized by an addition of lead and arsenic. The blend comprises about 8 percent by weight of the strontium chlorophosphate material, about 47 percent by weight of the zinc silicogermanate, and the remainder is a readily volatilized transport medium with a binder such as nitrocellulose. Of course with a different viscosity binder a greater or lesser percentage of binder will be preferred in preparing what is termed paint for coating. A layer 14 of this blend is coated onto the glass substrate, and this luminescent layer for a conventional 48 inch long, T-l2 soda-lime glass envelope comprises about 1.38 grams of the strontium chlorophosphate phosphor thoroughly blended with about 7.46 grams of the zinc silicogermanate phosphor.

The combination is now ready for application of a second layer. The second layer luminescent material which has a very good lumen maintenance characteristic, is for example, yttrium-europium oxide, which is a well known generally red emitting phosphor. A phosphor paint is prepared which is approximately ll percent by weight of phosphor with the remainder comprising a properly viscous transport medium with ethylcellulose binder. The viscosity of the paint is readily adjustable. A relatively thin relatively uniform top layer can now be applied over the first layer in a coating which for this size envelope comprises about 1 gram of the yttrium-europium oxide phosphor. The average particle size of the phosphor material used for the top layer is preferably less than the average particle size of the phosphor used for the first layer to insure effective coverage and protection of the first layer luminescent material. The coated envelopes are then heated to vaporize the transport medium and binders. The lamp manufacture can now be completed, forming a low-pressure mercury discharge fluorescent lamp.

In order to maintain distinct luminescent layers one must use as the vaporizable transport medium and binders materials which do not act to dissolve an already dried layer.

The effectiveness of the present double layered lamp is demonstrated by considering the following experimental data. The -hour lumen maintenance percentages for the phosphors described in the foregoing example practicing the present invention, measured when each phosphor was operated alone in a fluorescent lamp are respectively, about 67 percent for the strontium chlorophosphate, about 75 percent for the zinc-magnesium silicogermanate, and about 98 percent for the yttrium-europium oxide material.

The red, green, and blue lumen output figures for the lamp described above were obtained at 0-hour and lllhours using a Barnes calorimeter, which is a standard analytic tool. This device utilizes specifically absorptive filters over a properly responsive photocell. There are two filters designated amber X and blue X used to obtain the red lumen output. The filters are supplied by the Coming Glass Co. and the amber X, filter is designated CS 3l09, while the blue X, filter is CS 573. The green lumen output is determined using what is termed a Y filter designated CS 4-107 by Corning, and the blue lumen output by using a Z filter which is designated CS 5-72.

The maintenance of red light output from 0 hours to 100 hours was about 92 percent, for green about 89 percent, and for blue about 88 percent, with the maintenance for the total visible lumen output being about 94 percent.

In another example, where the ratios of the amount of yttrium-europium oxide, and the strontium chlorophosphate, and the zinc silicogermanate were varied the red, green and blue lumen maintenance figures for 100 hours were respectively 94 percent, 94 percent, and 88 percent, with a total lamp maintenance of 97 percent.

The 100 hour lumen maintenance value is a standard reference measurement point used in the industry, since a substantial portion of the lumen loss occurs typically in this time period.

While it is understood that there is a contribution from each of the phosphors and from the discharge to the other areas of the spectral energy distribution these maintenance figures do show that there was a considerable increase in the maintenance of the strontium chlorophosphate and the zinc silicogermanate by providing the protective layer of yttrium oxide-europium oxide.

It is, of course, desirable to determine the layer thicknesses and the phosphor particle densities such that the lumen maintenance for each of the phosphors used is approximately the same or as close to each other as possible to insure against color shifts of the lamp during long term operation.

As another example of the present invention, the first layer of luminescent material coated on the glass substrate is a blend of the green emitting manganese activated zinc silicate, and the red emitting cadmium borate, which are both characterized by poor maintenance. The second layer is formed of barium titanium phosphate which has a very good lumen maintenance. The ratios of these phosphors can be readily adjusted to provide a lamp with a generally white light output. The maintenance of the zinc silicate and the cadmium borate are both improved by the protective layer of barium titanium phosphate.

As yet another example, the first layer comprises cool white halophosphate phosphor, which is activated by well known proportions of manganese and antimony and a second layer comprises yttrium-europium oxide phosphor. The proportions of these phosphors used again is easily varied to vary the spectral energy distribution of the overall light output. The overall lumen maintenance is improved because the very high maintenance yttrium-europium oxide phosphor absorbs a high proportion of the very short wavelength ultraviolet radiation.

Another very high maintenance phosphor which lends itself to use as the top layer in a blend lamp according to the present invention is manganese activated calcium gallate. Other such compositions will be readily suggested to those skilled in the art. The relative amounts of the particular luminescent materials utilized in the plurality of distinct luminescent layers can be adjusted to provide a predetermined resultant spectral energy distribution.

lclaim:

1. In combination with a discharge lamp comprising a source of ultraviolet radiations which are directed upon a luminescent coating disposed on the inner surface of a light transmitting envelope which surrounds said source of ultraviolet radiations, and said luminescent coating comprises a plurality of different luminescent materials, the improvement which comprises:

said luminescent coating is disposed on said envelope as a plurality of distinct superimposed layers of different luminescent materials, a first of said layers coated directly on said envelope and comprising luminescent material which under prolonged and unscreened excitation by said ultraviolet radiations emits visible radiations of a first predetermined color but with a relatively poor maintenance of fluorescent response;

at least one additional layer of different luminescent material coated over said first layer, said additional layer of luminescent material under prolonged and unscreened excitation by said ultraviolet radiations emits visible radiations of a different predetermined color and with a relatively good maintenance of fluorescent response, said additional luminescent material layer serving to screen said first luminescent material layer from direct irradiation by said ultraviolet radiations, whereby the maintenance of fluorescent response of said first luminescent material layer and of said discharge lamp are both improved and the stability of the color emission of said lamp is improved.

2. The combination as specified in claim 1, wherein said additional layer has an average particle size which is less than the average particle size of said first layer, thereby insuring effective coverage and protection of said first layer.

3. The combination as specified in claim 1, wherein said combination comprises a low pressure mercury discharge device as said source of ultraviolet radiations and said layers are disposed on the interior wall surface of the envelope of said discharge device and said additional high maintenance luminescent material also protects said first layer from ion bombardment of the discharge sustaining gases.

4. The combination as specified in claim 1, wherein said source of ultraviolet radiations is a high pressure mercury discharge device having an outer envelope disposed about said discharge device, with said luminescent material layers being disposed on the interior wall surface of the outer envelope.

5. The combination as specified in claim 1, wherein said first layer comprises a blend of lead and arsenic stabilized manganese-activated zinc silicogermanate and europium-activated strontium chlorophosphate, and said additional layer comprises yttrium-europium oxide.

6. The combination as specified in claim 5, wherein the relative weight ratios of said silicogermanate, said chlorophosphate and said yttrium-europium oxide are approximately 7.46: 1.38: 1. 

2. The combination as specified in claim 1, wherein said additional layer has an average particle size which is less than the average particle size of said first layer, thereby insuring effective coverage and protection of said first layer.
 3. The combination as specified in claim 1, wherein said combination comprises a low pressure mercury discharge device as said source of ultraviolet radiations and said layers are disposed on the interior wall surface of the envelope of said discharge device and said additional high maintenance luminescent material also protects said first layer from ion bombardment of the discharge sustaining gases.
 4. The combination as specified in claim 1, wherein said source of ultraviolet radiations is a high pressure mercury discharge device having an outer envelope disposed about said discharge device, with said luminescent material layers being disposed on the interior wall surface of the outer envelope.
 5. The combination as specified in claim 1, wherein said first layer comprises a blend of lead and arsenic stabilized manganese-activated zinc silicogermanate and europium-activated strontium chlorophosphate, and said additional layer comprises yttrium-europium oxide.
 6. The combination as specified in claim 5, wherein the relative weight ratios of said silicogermanate, said chlorophosphate and said yttrium-europium oxide are approximately 7.46:1.38:1. 